The present invention relates to an electronic flash device.
There has been invented and used an electronic flash device which, instead of displaying by means of an indicator dial or panel, digitally displays an f-number and a speed of film used so that these exposure data can be easily seen and set to predetermined values by means of a shift switch.
In the case of the electronic flash device of the type described, the preselected f-number and film speed must be stored even when a main or power supply switch is turned off so as to stop a flash exposure temporarily. Therefore, the electronic flash device of the type described above is usually provided with an independent power supply which is not affected by the operation of the main or power supply switch. As a result, the electronic flash device of the type described inevitably becomes large in size because there must be provided a space for housing the independent power supply. In addition, because of the provision of the independent power supply, the cost of the electronic flash device becomes high.
In order to solve these problems, the same inventors have proposed an electronic flash in which the power can be normally supplied from a power supply of the electronic flash device to an integrated-circuit memory which stores therein predetermined exposure data such as an f-number and a film speed.
However, the power supply of the electronic flash device generally consists of a plurality of dry batteries. It follows, therefore, that when the main or power supply switch is closed when a main flash capacitor has not been charged yet, a conventional DC-DC converter is energized and consequently the voltage of the power supply drops suddenly to a low level. Therefore, when a common power supply is used for energizing the DC-DC converter and the integrated-circuit memory, the electronic flash device becomes advantageous in size and cost over the conventional electronic devices in which independent power supplies are provided for energizing the DC-DC converter to charge a main flash capacitor and for maintaining the exposure information stored in the integrated-circuit memory. However, when the voltage of the common power supply drops suddenly to a low level as described above, the exposure information stored in the integrated-circuit memory is erased.
In order to solve this problem, there has been proposed an electronic flash device in which a capacitor which is generally called a backup capacitor is connected across the common power supply in order to compensate for the voltage drop thereacross. However, the backup capacitor must have a high value so that the electronic flash device becomes large in size because it must house the backup capacitor and becomes also expensive to fabricate.